Bifocal contact lenses are lenses with two zones having different optical powers, or more than two zones in which case they are also called multifocal lenses. A far power zone provides the optical power that is needed for the wearer's distance vision and a near power zone, sometimes called a segment, provides the optical power that is needed for the wearer's near vision. The power zones together with an optional transition zone form the bifocal portion of the lens.
In order for a bifocal to function properly on the eye the pupil must be covered at least partly or completely by the far power zone of the lens when the eye observes a distant object and covered at least partly or completely by the near power zone of the lens when the eye observes a near object. This may be accomplished by a shifting action of the contact lens as the eye looks between distance and near. Alternatively, a principle known as simultaneous vision can be used in which the contact lens is designed and fitted in such a way as to position part or all of the far and near power zones in front of the pupil at the same time so that both contribute to the retinal image.
Bifocal contact lenses generally are classified into two types, segmented and concentric. Segmented bifocal contact lenses have two or more vertically separated power zones. Concentric bifocal contact lenses have two or more annular power zones.
There are several subtypes of segmented bifocal contact lenses, with dividing lines of different shapes between the power zones, including round, D shaped, flat, crescent, and others. The near power zone is placed at the lower portion of the lens and maintained in that position by one of various features that can be incorporated into the lens to stabilize the meridional rotation of the lens as much as possible. This is commonly accomplished by adding a "prism" to the lens, which thickens the lower area of the lens and serves to maintain the desired orientation and keep the near power zone of the lens downward relative to the pupil, making it ineffective during distance viewing. For reading or other near-vision tasks, the eye rotates downward and the contact lens shifts upward, placing the near power zone in front of at least part of the pupil and thereby providing an optical correction for near vision. Unfortunately, the shifting action is difficult to control, and may not occur when desired, which causes blurring of the wearer's vision. Shifting is especially difficult for soft contact lenses, compared to rigid contact lenses.
Concentric bifocal contact lenses have an advantage over segmented lenses in that they are usually more comfortable for the wearer and usually do not rely on a shifting action, although some shifting may be desirable. Many designs of concentric bifocal contact lenses exist, all of which have at least two power zones. They differ in terms of whether the power zones are spherical or aspherical and whether the far or near power zone of the lens occurs at the lens center. In addition, they differ in the design of the transition between the power zones.
Each bifocal lens type requires a different fitting procedure. It is generally felt that segmented bifocals are the most difficult to fit and this may contribute to their limited success. Concentric bifocals are easier to fit but have suffered from various optical problems that have limited their success.
Early concentric bifocal contact lenses such as the Bicon bifocal contact lens, described by Wesley and Jessen, and the DeCarle bifocal had two power zones connected directly with a junction but no transition zone. Several other concentric bifocal contact lenses are constructed without a transition zone as described, for example, in U.S. Pat. No. 4,636,049, U.S. Pat. No. 4,752,123 and U.S. Pat. No. 4,869,587. In each of these examples there is an abrupt change in slope at the junction of the far and near power zones of the lens. This change in slope causes a prismatic difference between the far and near power zones of the lens at the junction, which produces unwanted image jump or flare.
Various attempts have been made to smooth the junction between the far and near zones of bifocal contact lenses. One method of smoothing is by polishing, as described in U.S. Pat. No. 4,971,432 and U.S. Pat. No. 4,458,454. However, polishing of the junction is not an exact process and is subject to inconsistent results and reproducibility. The transition zone that is produced has an unknown curvature which cannot be described by a mathematical function and may not avoid prismatic difference between the far and near power zones of the lens.
Other methods to smooth the junction between the far and near zones of bifocal contact lenses involve using transition zones of various curves between the two power zones. Such a transition zone can be made in the form of a spherical curve as described in U.S. Pat. No. 5,864,379, an asphere as described in U.S. Pat. No. 5,864,379, U.S. Pat. No. 5,715,031, U.S. Pat. No. 5,754,270, U.S. Pat. No. 5,877,839, U.S. Pat. No. 5,125,729 and U.S. Pat. No. 5,798,817, a polynomial as described in U.S. Pat. No. 5,452,031, a spline as described in U.S. Pat. No. 5,452,031, or a slope angle as described in U.S. Pat. No. 5,608,471. These transition curves require special curve generating cutting equipment, generally lathes that are capable of multiaxis computer controls of the headstock and tailstock, as known to those familiar with the state of the art.
Some bifocal contact lenses have one or more power zones in the form of an asphere which may be designed to produce a smooth transition between the two power zones such as described in U.S. Pat. No. 5,754,270, which obviates the need for any transition curve. Unfortunately, it is found that when one or both segments of the concentric bifocal contact lens has an aspheric shape then that portion of the lens will not have an optimal optical effect. This occurs because in contact lenses, an aspheric curve will present a multiplicity of powers before the pupil at one time, which produces an imperfect image. Aspheric curves may also induce unwanted aberrations into the lens optics.
Still another method of smoothing the junction between distance and near power zones is to use materials of different indices of refraction for the two power zones. This produces a line-shaped junction which may have a smooth surface but has an abrupt change in prismatic effect when moving from one zone to the other as described in U.S. Pat. No. 3,726,587.